Near-infrared spectroscopy of H3+ above the barrier to linearity

Since the Royal Society Discussion Meeting on H-3(+) in 2000, the laboratory spectroscopy of H-3(+) has entered a new regime. For the first time, transitions of H-3(+) above the barrier to linearity have been observed. A highly sensitive near-infrared spectrometer based on a titanium: sapphire laser and incorporating a dual-beam, double-modulation technique with bidirectional optical multi-passing has been developed in order to detect these transitions, which are more than 4600 times weaker than the fundamental band. We discuss our recent work on the 2v(1) + 2v(2)(2) <- 0, 3v(1) + v(2)(1) <- 0, v(1) + 4v(2)(2) <- 0, v(1) + 4v(2)(4) <- 0 and 2v(1) + 3v(2)(1) <- 0 combination bands and the 5v(2)(1) <- 0, 5v(2)(3) <- 0, 5v(2)(5) <- 0 and 6v(2)(2) <- 0 overtone bands. Experimentally determined energy levels provide a critical test of ab initio calculations in this challenging energy regime (greater than 10 000 cm(-1)). By comparing the experimental energy levels and theoretical energy levels from ab initio calculations in which the adiabatic and relativistic corrections are incorporated, the extent of higher-order effects such as non-adiabatic and radiative corrections is revealed.